1. Field
Aspects of the present invention generally relate to a sheet conveyance apparatus that corrects skewing of a sheet, and an image forming apparatus that includes the sheet conveyance apparatus.
2. Description of the Related Art
A conventional image forming apparatus for forming an image on a sheet includes, in order to form an image on a sheet without any inclination, a skew correction device that corrects skewing of a sheet fed to an image forming unit.
Japanese Patent No. 4016621 discusses a skew correction device that corrects skewing of the leading edge of a sheet by causing the leading edge of the sheet conveyed by an upper roller pair disposed upstream of a registration roller pair to contact the nip portion of the registration roller pair and forming a loop on the sheet.
Japanese Patent Application Laid-Open No. 6-127753 discusses a skew correction device that corrects skewing by stopping a sheet conveyed by an upstream roller pair at an upstream stop position of the nip portion of a registration roller pair, resuming rotation of the upstream roller pair after a predetermined time has elapsed, conveying the sheet to the nip portion of the registration roller pair at a low speed, and forming a loop. According to the skew correction device discussed in Japanese Patent Application Laid-Open No. 6-127753, a sound or damage of the leading edge of the sheet generated or caused when the leading edge of the sheet fed by the upstream roller pair hits the nip portion of the registration roller pair can be reduced. In addition, the conventional skew correction devices include a type that has a function of setting the registration roller pair to a separated state and a press-contact state after the sheet, which is conveyed to the downstream side by the registration roller pair, has reached a transfer unit. This is because sheet conveyance during image transfer at an image forming unit needs to be carried out more accurately to improve image formation on the sheet. It is desirable to transfer an image while the sheet is conveyed only by a transfer roller instead of transferring an image while the sheet is nipped and conveyed by a plurality of rollers. There is also a type configured to correct positional deviation of the sheet in a width direction by moving the registration roller pair in the width direction of the sheet. The registration roller pair is moved to a home position while the registration roller pair is set to the separated state.
However, there is room for improvements of the conventional skew correction devices.
To reduce a sound or damage generated or caused when the leading edge of the sheet hits the nip portion of the registration roller pair, as in the case of the configuration discussed in Japanese Patent Application Laid-Open No. 6-127753, it is desirable that the sheet is stopped at a stop position upstream of the nip portion of the registration roller pair. In view of sheet productivity, the stop position of the sheet is desirably to be the position immediately before the nip portion of the registration roller pair.
To more accurately convey the sheet during the image transfer, the registration roller pair is desirably set to the separated state after the sheet has reached the transfer unit.
The following problems occur in the configuration where both the operation of stopping the sheet immediately before the nip portion of the registration roller pair and the operation of setting the registration roller pair to the separated state and the press-contact state are carried out.
For the registration roller pair set to the separated state after a preceding sheet has reached the transfer unit, a press-contact operation needs to be completed before the leading edge of a succeeding sheet contacts the nip portion of the registration roller pair. In a recent image forming apparatus, a sheet conveyance speed has become faster to improve productivity, and an interval between the preceding sheet and the succeeding sheet has become shorter. On the other hand, there is a limit to a speed (time) for setting the registration roller pair to the press-contact state.
Thus, in the configuration where both the operation of setting the registration roller pair to the separated state and the press-contact state and the operation of stopping the sheet immediately before the nip portion of the registration roller pair are carried out, the registration roller pair is set to a press-contact state after the succeeding sheet has reached the stop position and stopped at that position. Since the stop position of the succeeding sheet is immediately before the nip portion of the registration roller pair, if the skewing amount of the succeeding sheet is large, the leading edge of the succeeding sheet may be nipped.
Once the leading edge of the succeeding sheet has been nipped, the leading edge of the sheet cannot hit the nip portion of the registration roller pair to be aligned even if the upstream roller pair resumes its rotation to convey the sheet. This greatly deteriorates sheet skew correction performance. The deteriorated sheet skew correction performance causes formation of a defective image because the image cannot be formed in a normal position of the sheet.
The stop position of the sheet may be set at a position more upstream in the sheet conveying direction in view of a maximum skewing amount of the sheet so that the registration roller pair cannot nip the leading edge of the sheet reaching the stop position when the registration roller pair is set to the press-contact state. However, when the stop position is set more upstream in the conveying direction, much time is necessary for operations to resume the rotation of the upstream roller pair, convey the sheet at a low speed, and form a loop, consequently lowering productivity.